A portable headphone amp might be the simplest solution. For example, a "cmoy" amp that is housed in an Altoids mint tin can be had on Ebay for very little. That way you can avoid problems caused by the software methods, if there are any. Like, having less control at lower volumes, losing the boost when the software updates, etc. Also, it can be a little harsh when you've got the volume turned way up and it's too loud for the next situation you listen in. Granted, those may be less hassle than lugging yet another piece of techokit.